Medieval Skepticism

Overarching surveys of the history of philosophy often leave the
impression that philosophical skepticism—roughly, the position
that nothing can be known—had many adherents in the Ancient and
Hellenistic Periods, disappeared completely as a topic of intellectual
interest during the Middle Ages, and returned as a viable position in
the Renaissance and Early Modern Periods.

As a survey, this is quite understandable, since no thinker from the
Middle Ages professed an active allegiance to a systematic
philosophical skepticism. But a closer examination of Medieval
Philosophy shows that despite skepticism’s disappearance as an
overt philosophical movement, it continued to swirl in the thoughts of
many of the best philosophers of the period. A very few, including
most prominently Augustine and Al-Ghazali, claimed to have been
systematic skeptics at some points in their pasts. Many others held
skeptical views about localized issues such as one’s ability to
know an efficient cause. And even more discussed and attempted to
refute commonplace skeptical arguments in defense of their own,
anti-skeptical positions.

Chronologically speaking, skeptical issues were most prominently
considered in works from both the leading and tail ends of the Middle
Ages. Augustine’s 4th and 5th century attacks against the
Academic Skeptics mark the beginning of such discussions, and a
smattering of treatments of skeptical issues appears periodically
throughout the next 800 years. From the late 13th century onwards,
however, skeptical issues began to exert a dominant and wide influence
on epistemological discussions, as seen in the works of such important
figures as Henry of Ghent, John Duns Scotus, William of Ockham, Peter
Auriol, John Buridan, and Nicholas of Autrecourt.

Though medieval discussions of skepticism are often found buried
within larger, formulaic discussions of theological topics, these
treatments had influence beyond the academic circles within which they
were originally created and considered. Among Early Modern
philosophers, Descartes in particular owes a debt to these earlier
accounts of skepticism: versions of both his cogito and Evil
Demon arguments may be found in the works of medieval
philosophers.

In what follows we will briefly examine the relevant views of a few
representative figures from each tradition and era. Though none claims
to be inclusive of the entire Middle Ages, the best scholarly
overviews of important aspects of the medieval epistemological
tradition are Tachau (1988), Pasnau (1997), Perler (2006), and
Lagerlund (2010a).

There were many varieties of skepticism extant during the Ancient and
Hellenistic periods, but two were particularly important to the later
history of topic: Pyrrhonian Skepticism, especially as presented by
Sextus Empiricus, and the Academic Skepticism of Cicero.

Pre-medieval adherents of both types of skepticism not only held
particular skeptical positions, but also participated in a skeptical
way of life, taking their arguments and positions as part of an
overarching ethical worldview. Skeptics took their radical views of
knowledge as means to the end of reaching the state of tranquility. By
using common argumentative moves called tropes, skeptics sought to
elevate themselves and others to a state of suspension of belief
(epochê). And once this was reached, they held,
one’s worries about philosophical matters would dissolve in
tranquility.

Because of these ethical excellences, skeptics held themselves up as
wise men. The more radical Pyrrhonian Skeptics, who doubted the truth
of all claims, quickly ran afoul of the following objection, which has
been given in various forms throughout the history of philosophy: a
thoroughgoing skeptic, it seems, cannot live his or her skepticism. If
one doubts (and thus fails to act) on the truth of such claims as
“Food is necessary for human life”, it would follow that
one could not live at all. Academic Skeptics attempted to avoid this
objection by arguing that though skepticism precluded living by the
truth, since the truth could not be known, nevertheless one could live
by the truthlike or plausible. Hence theirs was a more practical
version of skepticism.

Pyrrhonian Skepticism, which was to play such an central role in the
Renaissance and Early Modern Philosophy, had no significant, direct
influence on later medieval thinkers, since texts exploring the
position (primarily the works of Sextus Empiricus, and to a much
lesser extent, Diogenes Laertius) were not in wide circulation.
Floridi (2002) and Wittwer (2016) explore the textual transmission of Sextus’s
works; Floridi notes that there are only seven known Latin manuscripts from
the period, though Wittwer has found further evidence to supplement this.

A few scattered references to Pyrrhonian skepticism have been found in
the Latin West, in the works of Bede (early 8th century), Rabanus
Maurus (9th century), and Peter of Auvergne (late 13th century). More
was known of it to Byzantine and Islamic philosophers, since knowledge
of Greek was preserved in their intellectual communities, and since
they had access to a greater range of ancient texts that addressed the
topic.

Academic Skepticism, so-called because of its birth among scholars
working in Plato’s Academy, was the type most known to the
medievals. Academic skepticism was presented through the sympathetic
works of Cicero (De Natura Deorum and Academica,
primarily), and especially through many of Augustine’s
anti-skeptical arguments, such as those found in his Contra
Academicos. In fact, for most of the Middle Ages—at least
up through the 1430s—the term scepticus wasn’t
used in the Latin tradition; academicus was the most common
term for the skeptic. Further complicating matters, the medievals
failed to recognize the distinction between Academic and Pyrrhonian
Skepticism. See Floridi (2002) and Schmitt (1972).

Schmitt’s (1972) study of the textual transmission of
Cicero’s skeptical works brings out many interesting aspects of
its history. As was the case with Sextus, there were few manuscripts
of Cicero’s Academica extant in the Middle Ages. In
addition, there were two versions of it in circulation, and the
medievals had only parts of each. And yet another problem for those
who had access to the texts was that Cicero’s position was often
confused with that of one of his interlocutors in the work,
Lucullus.

Henry of Ghent (late 13th C.) is the first medieval philosopher both
to have obvious knowledge of the Academica, and to have made
a serious philosophical attempt to come to grips with the views
expressed there. When John Duns Scotus critiques Henry’s
epistemology, he shows no evidence of knowing Cicero’s text. And
for the most part, later medievals were equally ignorant of it. Their
discussions of skepticism seem not to have been based on an
examination of or engagement with skepticism as presented by ancient
authors; it was a skepticism of its own sort, as will be detailed
below.

Augustine of Hippo (354–430) was a classically trained
rhetorician who explored many different schools of thought (Platonism,
Manicheanism, and Skepticism) before converting to Christianity. After
his conversion, he began to write philosophical and theological works
addressing some of the views from these schools. The most important
anti-skeptical work was his Contra Academicos (Against
the Academicians), which has been discussed by Matthews (1977 and
1992), Burnyeat (1982), King (1995), Curley (1997), O’Daly
(2001), Bolyard (2006), and Dutton (2016).

In Contra Academicos, Augustine targets a few key Academic
claims: (a) that appealing to truthlikeness or plausibility is
coherent; (b) that skeptics are wise; (c) that nothing can be known;
and finally (d) that skepticism leads to tranquility.

According to Augustine, three of the four claims can be relatively
quickly dispatched. The first claim, concerning truthlikeness, cannot
function alone as a standard, since one cannot know that something is
like the truth without also knowing the truth itself. Second,
skeptics cannot be wise, since wisdom requires knowledge of some sort.
Third, skepticism leads away from tranquility, rather than towards it,
since it puts one at odds with the morals of the rest of society,
which in turn is likely to lead to strife.

The most important claim for the epistemological history of the
problem is the third: that nothing whatsoever can be known. Augustine
treats of it in some detail.

He casts the issue as follows. The skeptic argues that a wise man must
retreat to skepticism since nothing can be known. This inability is
due to the fact that knowledge of a truth—at least as understood
by certain Stoics—is only possible if that truth could not
possibly be caused to appear mentally by something different than what
it is in fact caused by. For example, if an internal mental image or
concept of a tree’s being beside a house could be
caused by a dream, then the tree’s being beside the house cannot
be known, even if the tree is in fact beside the house. With
these stringent causal requirements, it is unsurprising to find that
Academic Skeptics take the line they do: since no appearance meets
this strict standard, they argue, it follows that nothing at all can
be known.

Augustine thinks this standard can be met, however, at least in some
cases. Augustine aims to uncover propositions about which doubt is an
utter impossibility. He soon finds the following four disjunctive
statements:

I still know something about physics. For I am certain that (1) there
is either one world or not. And (2) if there is not just one, the
number of them is either finite or infinite… In the same way, I
know that (3) our world is disposed as it is either by the nature of
bodies or by some plan. And I know that (4) (a) either it always did
exist and always will, or (b) it started to exist and will never stop,
or (c) it did not start in time but will have an end, or (d) it
started and will not last forever…These truths are [logical]
disjunctions, and no one can confuse a likeness of something false
with them. (Contra Academicos 3.10.23)

In short, Augustine challenges the skeptic to convince him that such
exhaustive, disjunctive propositions can be confused with, or have a
likeness of, what is false.

At this point that the skeptic counters with external world
skepticism: “How do you know this world exists…if the
senses are fallible?” In other words, the skeptic argues, these
disjunctive statements about the external, physical world all assume
the existence of an external world, and thus they cannot be known to
be true if the external world itself cannot be known to exist. If
external world skepticism can be maintained, it follows that
Augustine’s disjunctions can be mistaken for what is
false, and thus this particular argument against global skepticism
will fail.

Augustine’s primary response to the external-world skeptic is
Augustine’s claim that things “seem” to him, and
that these seemings constitute the world. He supports this view by
arguing that seemings are required in order for error to
occur—otherwise, what would we be mistaken about? And since the
possibility of error is the main impetus for skeptical doubt,
skepticism requires the admission that things seem. In other words,
for Augustine, one cannot doubt that one has mental content, even if
one might have doubt about whether this content corresponds to
anything external to the mind.

Augustine gives further, more central arguments against global
skepticism as Contra Academicos continues, claiming
mathematical truths (e.g., “2 + 3 = 5”) and logical truths
(e.g., “nothing both is and is not”) to be undoubtedly
true. As with the physical disjunctions, such truths can be known
without knowing external objects with any determinacy.

Beyond his discussions in the Contra Academicos, Augustine
frequently tackles epistemological topics in other works. Most
famously, Augustine makes proto-Cartesian moves frequently, arguing
that the mere fact that he doubts and has various other mental
happenings proves his own existence:

…who would doubt that he lives, remembers, understands, wills,
thinks, knows, and judges? For even if he doubts, he lives; if he
doubts, he remembers why he doubts; if he doubts, he understands that
he doubts; if he doubts, he wishes to be certain; if he doubts, he
thinks; if he doubts, he knows that he does not know; if he doubts, he
judges that he ought not to consent rashly. Whoever then doubts about
anything else ought never to doubt about all of these… (On
the Trinity 10.10.14)

Later, Augustine will draw on his theory of illumination to provide
the grounds for certainty. According to this theory, God’s
Divine Ideas serve as the guarantors of certainty, and they function
in much the way that Plato’s Forms do. Augustine first presents
this view in De Magistro (On the Teacher), and he
makes other references to it in later works. Augustinian Illumination
has been widely discussed in the secondary literature, and Nash (1969)
still remains one of the best introductions to the position.

There is little interest in skepticism exhibited in Christian
philosophy until the rise of the Universities in the 13th century.
Hadoardus (9th C.) includes many quotations from the
Academica in his compilation of Cicero’s views
generally, but he did no philosophical work with these quotations.
John of Salisbury (12th C.) discusses Academic Skepticism to some
degree in his Policraticus, but there’s no evidence
that he had direct access to Cicero’s text; he most likely got
the information either from Augustine or from some other secondary
source.

Two Islamic thinkers are particularly important to the history of
medieval skepticism. Al-Ghazali (Algazel to the Latin-speaking world)
(ca. 1058–1111) travelled throughout the Middle East, but spent
most of his time in what are now Iran and Iraq. Al-Haytham (= Alhazen)
(965–1039), who was born in what is now Basra, Iraq, wrote
widely on various scientific and mathematical subjects. In addition,
while the Persian philosophers Rāzī (1149–1210) and
Ṭūsī (1201–1274) are not skeptics, their
concerns with global skepticism and the knowledge of first principles
lead them to have extended discussions of skeptical arguments. For
more on Rāzī and Ṭūsī, see Fatoorchi (2013).

Al-Haytham’s Kitab al-Manazir (Book of Optics)
was of particular importance for the later history of skepticism.
Beyond his Arabic-speaking audience, it widely read in the Latin West
under the title Perspectiva or De aspectibus,
beginning with such philosophers as Roger Bacon (ca. 1214–1294).
His views about the perceptual process had a wide influence throughout
the later Middle Ages.

Al-Haytham held that many perceptions are inferential, and he explains
his views in II.3 of the Optics. Rather than always grasping
sensed things in an unmediated way, he argues, we sometimes grasp them
through sudden, “imperceptible” inferences. These
inferences proceed so rapidly as to seem immediate, and thus we
usually don’t notice that they are occurring at all. Al-Haytham
even argues that seemingly self-evident propositions such as
“the whole is greater than its [proper] part” are
inferential. Given this inferential process, cognitive error becomes a
more reasonable possibility.

He catalogued a number of optical illusions as well (Optics
III.7), examining such problems as the way the moon when low on the
horizon appears larger than it does when higher in the sky, and the
way that when one is in a boat floating down a river, the trees on the
shore appear to be moving. Though Al-Haytham was not a skeptic
himself, these illusory experiences provided fertile material for
later thinkers to consider. Tachau (1988) discusses his wide influence
on the scholastic tradition.

Al-Ghazali sounds surprisingly Cartesian in an important section of
his Munkidh min al-Dalal (Deliverance from Error).
He begins by declaring his desire to reach certain knowledge, which he
explains as “that in which what is known is laid bare in such a
way as to leave no room for doubt, and is unaccompanied by the
possibility of error or illusion, to the point that the mind cannot
even conceive it.” (Deliverance 82)

He gives a (by now) familiar list of reasons for doubting the
certainty of things. First, disagreement among competing theories
gives some initial doubt. Second, a few cases of sensory skepticism
(e.g., a shadow cast by the sun appearing to remain still, when in
fact it is slowly moving as the day passes; the apparently small size
of celestial bodies) lead him to lose confidence in all of his sensory
beliefs. This distrust of his senses also suggests, third, that
another of his faculties—reason itself—may be faulty, and
he wonders whether even apparent logical truths might be false. And
finally, he concludes by invoking dream skepticism. After setting up
these doubts, he says the following:

When these notions occurred to me and made an impression on my mind, I
sought a cure but found none. For they could only be rebutted with a
proof, and a proof can only be constructed by combining the first
[principles of] knowledge. If these are not given, then it is
impossible to arrange a proof. This disease defied all cure and lasted
for almost two months, during which I embraced the [skeptical] creed
in actual fact, though not in speech or expression. Eventually, God
cured me of this disease and my mind was restored to health and
balance. The rational necessary beliefs were once again accepted and
trusted, both securely and certainly. This did not come about by
composing a proof or by an arrangement of words, but rather by a light
that God almighty cast into my breast, which is the key to the greater
part of cognizance. Whoever supposes that enlightenment depends upon
explicit proofs has narrowed the expanse of God’s mercy.
(Deliverance 86)

Beyond this, Ghazali also questions the nature of causation in his
Incoherence of the Philosophers (Tahafut
al-falasifa). Though he ultimately holds that all causation can
be traced to God, he argues that our observations of so-called natural
causes are not sufficient for proving a direct causal link between the
apparent cause and that which is caused. This Humean-leaning position
has been discussed widely in the secondary literature. See, e.g.,
Halevi (2002) for a recent treatment. For another recent account that
does much to situate Ghazali’s discussion of skepticism within a
broader Islamic intellectual conversation about the subject, and
downplays the supposed connections between Ghazali and the Early
Moderns, see Kukkonen (2010).

There is no strong evidence of any significant skeptical tendencies or
interests among medieval Jewish philosophers. Judah Halevi (ca.
1075–1141) discuses skepticism briefly in his Kuzari
I.4–8; in this passage, a character in the poem professes
skepticism about religious truths, and presents his requirements for
what would count as knowledge. See Kogan (2003).

There has also been limited discussion of Maimonides as a skeptic.
Some of it focuses, e.g., on his claims in the Guide for the
Perplexed 2.24 that humans cannot have knowledge of heavenly
things. To take this to imply either a thoroughgoing skepticism or a
thoroughgoing concern with skepticism, however, is probably too strong
an inference. For more on this issue, see Ivry (2008).

The thirteenth century saw the birth of Scholasticism in the Latin
West. As Universities began to develop in such important centers of
learning as Paris and Oxford, so too did highly formalized and
argumentative styles of debate and writing. At the same time, some of
the intellectual consequences of the Crusades came to play an
important role in the history of skepticism: Muslim and Jewish
scholars and writings came to the attention of Christians working on
similar topics. Of particular importance was the translation of all of
Aristotle’s works into Latin, along with many commentaries on
them (as well as original works) by Ibn-Rushd (Averroes) and Ibn-Sina
(Avicenna).

With these texts came others (such as Al-Haytham’s
Optics), and Christian scholars such as Roger Bacon began to
investigate the cognitive process more thoroughly in their own
writings. The dominant Augustinian theory of knowledge began to come
under attack as the wealth of new accounts were contrasted, rejected,
or synthesized. And as Augustine was reinterpreted, so too was his
rejection of skepticism.

Thomas Aquinas (ca. 1225–1274) and Siger of Brabant (ca.
1240–ca. 1282) were philosophers of vastly different reputations
(the first was declared a saint and holds a preeminent place in
Catholic theology; the second was accused of heresy and died under
mysterious circumstances). Yet they both shared a deep commitment to
synthesizing the new Aristotelian texts into their respective
views.

Aquinas, as with Aristotle, exhibits no serious concerns with
skepticism or with skeptical arguments. He occasionally makes
references to sensory illusions, e.g., but he sees them as no
epistemological threat. Baertschi (1986) and Pasnau (1997) treat of
this issue briefly. Indeed, most of the secondary literature on
Aquinas focuses on the question of why he has no such
interest in skepticism. Varying accounts are given, and among them is
Aquinas’s Aristotelian belief that the cognitive process is
fundamentally a reliable one. For the most part, Aquinas and most
later medievals aim to explain the processes by which knowledge is
acquired, rather than aiming to justify knowledge.

Furthermore, many scholars argue that the Aristotelian doctrine of the
formal identity of knower and known plays a significant role for
Aquinas in particular. If (on this interpretation) the knower quite
literally takes on the form of the known object, and thus becomes
identical to the known object in this formal way, then there is no
chance for error. The knower is not at a remove from the known object
at all, on this account. There is considerable disagreement about
Aquinas’s motivations here; for a few representative views, see
Gilson (1986), MacDonald (1993), Pasnau (1997), Jenkins (1997), and
Hibbs (1999).

Siger of Brabant, on the other hand, dealt directly with skepticism
and skeptical arguments in his Impossibile 2 and his
Questions on the Metaphysics. Though, as Côté
(2006) argues, he also declines to take skepticism to be a serious
threat, he does take the time to address it. Most notably, Siger
raises the following question for consideration in
Impossibile 2: “everything that appears to us are
illusions and similar to dreams, so that we are not certain of the
existence of anything.” Siger has various responses in his
discussions, but his most important claims are (a) that a failure of
the senses in some cases does not automatically imply failure in all
cases; and (b) that if a sense report is not contradicted by another,
more reliable sense report, then it itself is reliable. Furthermore,
Siger gives a rather unconvincing reductio, arguing that if
the senses are unreliable, no knowledge at all is possible. Taking
this as a reductio of skepticism obviously would do little to
assuage the worries of the committed skeptic.

Siger’s responses, though somewhat unsatisfying, do indicate the
beginning of a growing interest in skeptical problems. Henry of Ghent
shows this interest even more starkly.

Henry of Ghent (ca. 1217–1293) was one of the most important
theological masters of his day, and he was a contemporary of both
Aquinas and Siger. Beyond his own philosophical work, Henry was a
central figure in one of the crucial events in medieval intellectual
history: the Condemnation of 1277, which will be discussed at the end
of this section. Brown (1973), Marrone (1985), Pasnau (1995), and
Adams (1987) discuss Henry’s views in some detail.

Henry’s most concentrated attention to skeptical issues occurs
in the first two questions of his Summa Quaestionum
Ordinariarum (Ordinary Questions). Henry’s
discussion of skepticism stands out when placed alongside other works
from the same period. Though Augustine’s Contra
Academicos was extant, and though Augustine’s De
Trinitate echoed many of the anti-skeptical arguments from his
own earlier work, the vast majority of Henry’s scholastic
contemporaries (including Aquinas) took no serious interest in
skepticism. Various explanations of this general attitude can be
given. Perhaps Augustine’s self-proclaimed refutation of
Academic skepticism was taken to be the final word on the subject;
Aristotle’s dismissive attitude towards skepticism would have
reinforced this idea. But for whatever reason, Henry thought the issue
of skepticism important enough to raise it in the opening question of
his own most important theological work.

Henry lists a number of different skeptical arguments, drawing from
the critical accounts of Aristotle, Cicero, Augustine, and Averroes,
and mentioning the support skepticism garners from the views of
Heraclitus, Zeno, Protagoras, and Democritus, and Plato. He gives no
evidence here of having direct access to any of the texts of the
latter five thinkers, though he knows of their views through the works
of others.

He begins by listing preliminary arguments both for and against the
possibility of knowledge. On the skeptic’s side, Henry discusses
cases of sensory relativism (what seems sweet to one person does not
seem sweet to another, e.g.); the changeable nature of the sensory
world; and the Learner’s Paradox from the Meno. Among
the anti-skeptical arguments is Aristotle’s view
(Metaphysics IV) that in denying knowledge, one is thereby
claiming certainty that one does not know, and thus the skeptic must
admit to knowing something. He also pulls from Augustine’s
oft-repeated claim that in doubting, one knows that one doubts (De
vera religione xxxix.73).

Henry then argues in a number of different ways that knowledge is in
fact possible. First, he draws from Augustine and Cicero. His weakest
claim here is that we can rely upon the testimony of others;
otherwise, he says, knowledge of the distant past, or of places that
one has never visited, would be impossible. He also explains that one
can trust the veracity of a given sense experience provided it
hasn’t been contradicted by a more reliable sense experience. In
addition, he says that even if one is dreaming, one still knows that
one lives. As with many who follow him, Henry cites the certainty of
the law of non-contradiction as well.

In the final section of the question, Henry replies directly to the
skeptical arguments he outlined in the beginning. Though he gives too
many responses to detail here, Henry’s core idea is that though
the senses grasp only changeable things, one has the ability to
abstract what he calls the “created exemplar” from the
objects of the senses; from this created exemplar, we can obtain a
low-level knowledge of external objects (he calls this knowledge of
the “true” or of the “truth”). Knowledge in
the full sense—that is, knowledge of the “pure
truth”—requires knowledge of the “uncreated
exemplar”, or Divine Idea. Because the created exemplar is
mutable in itself, it is only by seeing how it accords with the
uncreated exemplar in God’s mind that full and certain knowledge
is possible. In short, Henry follows Augustine in spirit, even if not
in detail: for both philosophers, knowledge is impossible without
Divine Illumination.

In the second question of his Summa, Henry explores
Illumination in more detail. As he begins to explain things, it sounds
as if God’s general background influence is sufficient to
explain human knowledge. Later, however, Henry limits his optimistic
outlook. First, he argues that God illuminates each person

according to his condition and capacity, unless someone by displaying
great malice merits that it be taken away from him altogether. Such a
person, as a result, would not see any truth at all…but would
dissipate into the error that he deserves. (Summa I.2.134)

Echoing some of Augustine’s remarks in the De Magistro,
Henry here seems to restrict epistemic certainty to those who are
morally worthy. Second, Henry diverges even further from his initial
argument, saying that God offers the “rules of the eternal
light”—that is, the Divine Ideas—

to whomever he wants and takes them away from whomever he
wants… Thus God sometimes bestows the eternal rules on bad
people, with the result that in these rules they see many truths that
the good cannot see… Sometimes, too, God takes these same rules
away from such people and allows them to fall into error… [God]
bestows [pure truth] through free will, on whomever he wants.
(Summa I.2.131–132)

In short, according to this second argument, our ability to know with
certainty is entirely dependent upon God’s whim. We will know
only in cases in which God wants us to. This emphasis on God’s
role in the knowing process is of a piece with the emphasis on Divine
Omnipotence one finds in the Condemnation of 1277, with which Henry
was intimately involved.

As the newly rediscovered Aristotelian texts began to find their way
into university curricula in the thirteenth century, more conservative
faculty reacted. Bonaventure and Henry were among the latter, and each
argued against those who sought to replace the reigning Augustinianism
with too many new Aristotelian elements. Aquinas, Siger of Brabant,
and others sought to synthesize Aristotle and Christianity in a much
more thoroughgoing way than Henry thought acceptable. And as part of
the commission organized at the Pope’s request, Henry helped
create a list of 219 propositions—some held by Aquinas
himself—that were condemned as heretical to the Catholic faith
in 1277 by Bishop Etienne Tempier.

If there were ever an instance of philosophical irony in the Middle
Ages, this would be it. Despite Henry’s strong aversion to
skepticism, and despite his arguments against it, the most important
practical effect of the Condemnation of 1277 was to introduce an
entirely new level of skeptical doubt. The Condemnation emphasized
God’s omnipotence, and declared views that denied this to be
heretical. As a result, the realm of the possible was expanded
dramatically in medieval discussions. This concern quickly spreads
throughout most Christian epistemological discussions, up through the
end of the Middle Ages. If God is omnipotent, according to this
concern, couldn’t he be deceiving us either in particular cases,
or perhaps even globally? For a fascinating discussion of the variety
of responses one finds in the 13th and 14th century treatments of this
problem, see Perler (2010).

After the Condemnation of 1277, Christian philosophers became even
more focused on epistemology. Debates often centered on the medieval
distinction between intuitive cognition and abstractive
cognition—roughly, the distinction between knowing something as
present and existent, and knowing something from a remove (e.g.,
through memory, or through an inference). In addition, many
philosophers began to explore the nature of sensory illusions in more
detail. And of course, the Evil Demon hypothesis loomed ever larger as
the notion of Divine Omnipotence was explored more fully.

John Duns Scotus (1265–1308) worked in Oxford, Paris, and
Cologne. Living roughly a generation before Ockham, Scotus was a
follower of Aristotle, and as with many of his time, Avicenna too had
a profound impact on the development of his thought. As far as
skepticism is concerned, Scotus is unconvinced by Henry’s
anti-skeptical arguments, but he thinks the threat of skepticism
dangerous enough that he devotes considerable attention to arguing
against the problem. Adams (1987) and Pickavé (2010) discuss
his position in connection with skepticism.

In his Ordinatio I.3.1.4, Scotus finds Henry’s created
exemplar/uncreated exemplar distinction insufficient for defeating
skepticism. Scotus’ critique of Henry has two main foci:
Henry’s appeal to mutability, and Henry’s need for an
uncreated exemplar. First, Scotus finds numerous problems with
Henry’s worries about change, and he argues that change as such
does not prevent knowledge, and that even if it did, much of what we
know is sufficiently stable to support our knowledge claims. In
defense of his initial claim he argues, e.g., that our own mutability
would make knowledge utterly impossible, if Henry’s views are
correct. His second claim about change also receives support in
various ways, most notably by his appeal to what he calls a nature
(natura), which is (roughly) the essence of a thing. Here, he
argues that since natures in themselves are immutable, and since each
can have what Scotus calls an immutable relation to something else, we
have sufficient grounds for stability-based certainty.

Henry’s appeal to an uncreated exemplar to ground knowledge and
certainty is also problematic, according to Scotus. If we understand
the created exemplar as a species (roughly, an image or intentional
object) formed in the soul during an act of cognition, then we are
often unsure whether that created exemplar existing in the soul truly
corresponds to an extramental object. Thus,

…if it cannot be judged when such a species represents itself
as such and when it represents itself as an object, then [no matter]
what else concurs with such a species, one cannot have [any] certitude
by which the true may be distinguished from the truthlike.
(Ordinatio I.3.1.4.104)

In other words, showing that the species in the soul corresponds to an
uncreated exemplar—that is, a Divine Idea—does
nothing to help us determine whether that species corresponds to
something in the sensory world.

According to Scotus, God has created the world in such a way that
knowledge is possible by means of his general, background
illumination, which amounts, in Scotus’ view, to a natural
process. With this in mind, we may now turn to an examination of
Scotus’ positive view and its relation to skepticism.

Scotus holds that we have “necessary certitude” about four
kinds of knowledge. The first type is knowledge of self-evident
propositions (propositions per se notae)—such as
‘a whole is greater than its parts’—as well as
knowledge of propositions derived syllogistically from them. This type
of knowledge amounts to necessary, analytic truths, in his view: once
one knows the terms that enter into such a proposition, and once those
terms are combined into the proposition, one cannot help but assent.
Scotus’ second type of knowledge is knowledge of our own
contingent acts, including such propositions as ‘I am
awake’ and ‘I am alive’. Scotus follows Augustine in
holding that such knowledge is immune to skeptical attack because even
if the senses are deceived, once these terms are grasped, we can know
the truth about them in such propositional contexts.

Though much can be said about these types of knowledge, the most
relevant discussions for our purposes deal with the remaining types.
Our certitude here depends crucially on the following claim:

Whatever happens frequently through something that is not free, has
this something as its natural per se cause.
(Ordinatio I.3.1.4.106)

In other words, Scotus suggests a general inductive principle:
whenever something occurs frequently over time, such repeatability
cannot be due to chance. God has ordained that such regularities will
occur, and thus we can reach a general principle based on those
initial cases. Such regularities amount to natural occurrences, and
thus require no appeal to special illumination.

Given this, his third type of certainty is discussed: what Scotus
calls things knowable “through experience”—e.g.,
that “a certain species of herb is hot”. Such general
claims, derived through our experience of numerous instances of the
hotness of such herbs, are certain in virtue of the “non-free
cause” principle above. Recognizing, however, that inductions
don’t hold the same level of assurance that he is claiming for
first two types of knowledge, Scotus backs off of his claim a bit
later, calling it “the lowest degree of scientific
knowledge”, and admitting that such inductions may only tell us
that such regularities are “aptitudes”, not certainties
(Ordinatio I.3.1.4.110–111).

When Scotus begins discussing his fourth type of
certainty—particular knowledge claims about the external world,
known through the senses—he ignores this weakened conception of
our senses’ reliability. Though later thinkers will be clearer
in their indebtedness to the Condemnation of 1277 here, Scotus gives
minimal notice of this. Instead, appealing again to his non-free cause
claim, he gives explanations of two main types of such experience.

First, because it is often the case that different sense modalities
agree in their judgment about an external object—e.g., when we
can both touch and see the size of a ball—we have an induction
of sorts running here, and thus we can infer that this regularity is
enough to give us certainty regarding the object under
consideration.

Second, in cases in which the sense modalities are not in
agreement—either because one modality yields a different result
than another modality, or because a single modality yields different
results at different times—we can appeal to the intellect to
adjudicate among them. Using his example, we know that a stick in
water that appears broken cannot really be broken, because our
intellect knows the truth of the claim ‘the harder object is not
broken by the touch of something soft that gives way before it’
(Ordinatio I.3.1.4.114–115). Thus, in such a case, we
can discount the testimony of sight. Scotus makes a similar move
regarding the apparent deception that occurs in dreams. In his view,
“a person can tell when his faculty is disposed and when it is
not”, and thus he can tell whether he is asleep or dreaming
(Ordinatio I.3.1.4.118–120).

Peter Auriol (1280–1322) and William of Ockham (1285–1347)
were contemporaries, though they took different paths both
philosophically and ecclesiastically. Auriol spent most of his time at
the University of Paris, and eventually became an Archbishop before
his untimely death. Ockham studied and taught at Oxford before being
brought up on charges of heresy by the papal court in Avignon; he
spent the last years of his life excommunicated from the Church, after
having fled to Munich. Though there is no evidence of the two having
ever met, Ockham often argues against Auriol’s views in some
detail. Adam Wodeham (ca. 1300–1358), who commented on both of
their views, was the personal secretary of Ockham for a time, and
worked at Oxford.

Auriol’s role in the history of skepticism is twofold, and he
has been discussed in this connection most recently by Tachau (1988),
Perler (1994), and Denery (1998). First, he develops an account of
intuitive cognition that raises the possibility of sensory illusion;
second, he discusses particular cases of sensory illusion in some
detail in his Scriptum (prologue, q. 2 and d. 3, q. 14).

He begins by diverging from Scotus’s account of cognition.
Scotus suggests that cognition of God, and cognition generally, can
occur in one of two ways: either abstractively or intuitively.
Intuitive cognition is meant to include a human’s more-or-less
direct sensory experience of the external world. Abstractive
cognition, on the other hand, is knowledge from a distance; it
abstracts from the presence and existence of the thing, as when we
remember a deceased acquaintance or perform astronomical calculations
in a windowless room.

Auriol agrees with much of Scotus’s account of intuitive and
abstractive cognition. Yet he imbues it with a psychological character
that is absent in the latter’s work. For Auriol an intuitive
cognition is had when one has the experience of something
as if it is present and existent. It is even possible, in
Auriol’s view, to have such a cognition when the thing itself is
absent or non-existent. Auriol’s abstractive cognition, on the
other hand, does not involve this experience or feeling of
something’s presence and existence, even if the thing
is both present and existent. For any given state of the
extramental world, both abstractive and intuitive cognitions can
occur. As a result, his position leaves him open to skeptical
attack.

He realizes this possibility, and discusses many illusory experiences
before developing a response. These illusory experiences include such
stock examples as dreams, hallucinations, mirror images, the
after-images of the sun, the bent appearance of a straight stick that
is immersed in water, and the apparent motion of trees experienced by
those traveling down a river. He also mentions such cases as the
double image of a candle that appears when one’s eyes are
distorted, the shimmering, changing appearance of colors on a
dove’s neck, and most interestingly, the fiery circle that
appears when a burning stick is whirled rapidly through the air.
Though Auriol’s discussion stresses some experiences more than
others, his basic point is that failing to identify such events as
intuitive cognitions amounts to the assertion that “all things
that appear, are” (Scriptum 3.14.697).

Auriol responds to these challenges by distinguishing between real
being (esse reale) and apparent being (esse
apparens). This distinction has perplexed most readers of Auriol,
and there is considerable disagreement about how to interpret it. Even
so, it is generally agreed that real being is what the object has
independently of any perceiver, and also that whatever it is that is
meant by esse apparens, it is to be identified with a mental
or sensory appearance of some sort. Some scholars (e.g., Tachau) read
Auriol as a representationalist, which of course does little to solve
the skeptical problem; others (e.g., Perler) see him as a direct
realist. Whatever the answer in this particular case, Auriol is no
skeptic. Not only does he believe that we can know external objects;
we also know many self-evident propositions with certainty (logical
truths, e.g.). For more on this aspect of Auriol’s thought, see
Bolyard (2000).

William of Ockham considers Auriol’s perceptual problems, but he
concludes that they are not a serious threat. On his view, our
perceptual process (which occurs by means of intuitive cognitions) is
such that it is infallible: for any such intuitive cognition, if it is
of a thing that exists, we will know this fact, and if it is not, we
will know this as well. He holds this view even given the possibility
that God is deceiving us about such perceptions (e.g., by destroying
the object while maintaining the perception of it).

Adam Wodeham disagrees with Ockham on this point; for him, there is no
clear mark by which we can distinguish a true perception from a false
one in the case of a deceptive God. Nevertheless, he holds that our
perceptual process is generally reliable despite these problems. For
more on Ockham and Wodeham, see especially Karger (2004), Panaccio and
Piché (2010), and Wood (2003); Adams (1987) and Tachau (1988)
also discuss their skeptical and anti-skeptical views.

William Crathorn (fl. 1330) was not considered by his contemporaries
or later medieval commentators to be of the stature of such thinkers
as Aquinas, Henry of Ghent, Scotus, or Ockham; still, his views give a
window into some of the skeptical worries extant at the time. He
worked at Oxford, flourishing in the generation after Scotus, and
during the time of Ockham. Tachau (1988) and Pasnau (1997) discuss his
views.

In his Questions on the First Book of Lombard’s
Sentences, q.1, the Condemnation-inspired acknowledgment of
God’s omnipotence generates and reinforces many skeptical
problems for Crathorn. In response, Crathorn uses God to bring himself
back from the skeptical abyss. Though not as rhetorically compelling
as Descartes’ analogous moves in the Meditations, the
philosophical similarities among the two are striking.

Crathorn also makes frequent appeal to God’s omnipotence and
power to deceive us—nearly every page makes reference, directly
or obliquely, to this possibility. A favorite non-epistemological
example he uses concerns heat and fire: God, he repeatedly says, has
the power to separate the heat from the fire that normally produces
it. He also extends such divine powers to sensory cases. God, it
seems, could maintain the vision of something even after that thing
ceases to exist. And as he tells us later, God’s power to do
this is vast. Here, his example is that of the lighted, fiery circle
we see when a torch is rapidly twirled through the air at night:

…if God were to preserve in your head for a whole year that
circular color or another like it while no color existed externally,
it would appear to you seeing that circular shape that you were seeing
for the whole year a flaming circle and the color of a circular shape
existing outside you—when nevertheless there was no such thing.
(Questions I.98–99)

Similar examples are used to show that we can be deceived in other
ways as well. Afterimages of colors can remain briefly after
we’ve turned away from that which caused the initial color
sensation. And it is within God’s power both to preserve a
sensible species of color after destroying the thing, and even to
create such a sensible species even when no extramental thing ever
existed. And finally, he mentions dream skepticism. Unlike Scotus and
most others who discuss this problem, Crathorn explains a case in
which one who is awake thinks he is dreaming.

It is here that Crathorn begins to move us out of our skeptical
position, by putting limits on God’s power to deceive. First, he
shows us cases in which God’s power cannot extend to the
logically contradictory: even God, Crathorn says, cannot make a stone
think. Second, he agrees with Scotus that seeming claims (e.g.,
‘I feel hot’) and standard self-evident propositions
(e.g., ‘the whole is greater than its part’) cannot be
doubted. Furthermore, he follows Augustine in arguing that this
inference cannot be doubted: ‘I doubt that I exist; therefore, I
exist’.

For more standard sensory skepticism, however, he combines the
approaches offered by Henry and Scotus. By appealing to a self-evident
proposition concerning God’s goodness, Crathorn tells us, we can
show that external world skepticism is incoherent. A benevolent God
would not systematically deceive us in this way.

Nicholas of Autrecourt (ca. 1300–ca. 1350) and John Buridan (ca.
1295–1361) were contemporaries at the University of Paris. While
Buridan maintained a good relationship with his ecclesiastical
superiors, Nicholas did not: the latter’s works were condemned
and publicly burned. Of particular interest in what survives are two
of his Letters to Bernard of Arezzo. Recent discussions of
Autrecourt’s views may be found in Beuchot (2003), Zupko (2003),
and Grellard (2007).

In his First Letter, Autrecourt argues that Bernard’s
views lead to an extreme form of skepticism. As he interprets the
view, it would follow that intuitive cognitions cannot guarantee their
own certitude: sensory illusions and the possibility of a deceptive
God preclude this. But he goes further. As he explains, “you
must say that you are not certain of your own acts, for example, that
you are seeing or hearing”. Furthermore, “you are not
certain whether anything appears to you at all” (First
Letter 11). In short, one cannot be certain about any aspect of
the external world, including even its very existence. And as he goes
on to say, the existence of the past is equally uncertain, as is the
very existence of one’s own mind.

Autrecourt’s Second Letter seeks to temper this
skepticism, but only to a degree. According to him, the only things of
which we can be certain are the principle of non-contradiction (i.e.,
“nothing both is and is not”) and other propositions that
can be derived from this principle. He maintains a causal,
proto-Humean skepticism about existential inferences: “From the
fact that some thing is known to be, it cannot be inferred evidently,
by evidentness reduced to the first principle, or to the certitude of
the first principle, that there is some other thing” (Second
Letter 11). As he continues, he says that the only substance of
which we can possess evident knowledge is his own soul

Nicholas of Autrecourt espoused the most radical form of skepticism
found at any point during the Middle Ages, and he was punished for it.
Buridan, however, argues specifically against Autrecourt in his own
works.

In his Questions on Aristotle’s Metaphysics II.1, for
instance, Buridan discusses various skeptical challenges, including
sensory illusion, dream skepticism, skepticism about induction, and
Autrecourt’s causal skepticism. Again, the deceptive
possibilities of an omnipotent God play a large role in his worries
here.

In response, Buridan takes a few different approaches. First, as with
Autrecourt, Buridan holds the principle of non-contradiction to be
undeniable, as is every proposition that can be derived from it. But
he also says that there is a “virtual infinity of self-evident
principles through the senses, or through experience, or through the
inclusion of terms without having to be proved by means of the first
principle [i.e., non-contradiction]” (Questions
II.1.147, Klima trans.). In addition, Buridan drops his
epistemological standards for sensory knowledge in general: because of
the possibility of God’s deceptiveness, at best we are capable
of “conditional evidentness”. Similar reductions in
standards occur in cases of induction, causation, etc. As he says,
mathematical certainty is not expected in every subject. For more on
Buridan and his broader intellectual context, see Zupko (2003),
Grellard (2007), Lagerlund (2010b), and Karger (2010).

Medieval Skepticism was not a movement. Rather, it was a series of
(sometimes isolated) worries and responses to such skeptical problems
as those outlined above. While some impetus for later discussions was
gained from classical skeptical sources, for the most part medieval
skepticism took its own path. Among the distinctly medieval additions
to the debate were an emphasis on the certainty of self-knowledge, and
especially on a widespread recognition across traditions that
God’s omnipotence, and thus the possibility of Divine deceit on
these grounds, provides a special challenge to the epistemology of
anyone who holds a theistic worldview.

The fate of skepticism in the Renaissance and Early Modern Periods has
been discussed widely, but connections between these later versions
and those of their medieval antecedents have been less thoroughly
studied. Heider (2016) explores skeptical themes in the “Second
Scholasticism” of the 16th and 17th centuries. Thinkers such as
Francisco Suárez, John Poinsot, and Francisco de Oviedo
continue to treat the Scotistic/Auriolian/Ockhamist issue of the
intuitive cognition of non-existent objects. They do not consider
global skepticism a live threat, as Descartes does, and their accounts
are thus closer to those of 13th and 14th century philosophers.

For an overview of the later history of the skepticism, with a focus
on canonical Early Modern philosophers, see Popkin (2003).

Note: Texts in this section are alphabetized according to the
first name of later medieval Latin authors, according to
scholarly convention. Hence “William of Ockham”, e.g., is
listed in the Ws, not the Os.

Karger, E., 2010, “A Buridanian Response to a Fourteenth
Century Skeptical Argument and its Rebuttal by a New Argument in the
Early Sixteenth Century,” in Rethinking the history of
skepticism : the missing medieval background, H. Lagerlund (ed.),
Leiden: Brill Publishing, pp. 215–233.

Klima, G., 2010, “The Anti-Skepticism of John Buridan and
Thomas Aquinas: Putting Skeptics in Their Place versus Stopping Them
in Their Tracks,” in Rethinking the history of skepticism :
the missing medieval background, H. Lagerlund (ed.), Leiden:
Brill Publishing, pp. 145–170.

Kogan, B. S., 2003, “Judah Halevi and his use of philosophy
in the Kuzari,” in The Cambridge Companion to
Medieval Jewish Philosophy, D. Frank and O. Leaman (eds.),
Cambridge: Cambridge University Press, pp. 111–135.